body {
    width: 100%;
    height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    background-color: rgba(255, 155, 233, .5);
}

#app {
    position: relative;
    width: 800px;
    height: 450px;
    border: 5px solid rgb(253, 115, 223, .5);
    user-select: none;
}

#app:hover a {
    opacity: 1;
}

#app img {
    height: 100%;
    width: 100%;
    position: absolute;
}

#app a {
    display: block;
    height: 100px;
    width: 50px;
    background-color: rgba(0, 0, 0, 0.5);
    font-size: 30px;
    color: #fff;
    text-decoration: none;
    text-align: center;
    line-height: 100px;
    position: absolute;
    z-index: 99;
    top: calc(50% - 50px);
    opacity: 0;
    transition: opacity 1s;
}

#app a:nth-child(1) {
    left: 0;
}

#app a:nth-child(2) {
    right: 0;
}